"one who bears testimony to faith," especially "one who willingly suffers death rather than surrender his or her religious faith," specifically "one of the Christians who in former times were put to death because they would not renounce their religious beliefs," Old English martyr, from Late Latin martyr, from Doric Greek martyr, earlier martys (genitive martyros), in Christian use "martyr," literally "witness." Sometimes said to be related to mermera "care, trouble," from mermairein "be anxious or thoughtful," from PIE *(s)mrtu- (source also of Sanskrit smarati "remember," Latin memor "mindful"), however Beekes has phonetic objections to this and suggests it is rather a loan-word from Pre-Greek.
Adopted directly into most Germanic languages, but Norse substituted native formation pislarvattr, literally "torture-witness." General sense of "constant sufferer" is from 1550s. For sense shift from abstract "testimony" to "a witness," compare French témoin "witness" from Latin testimonium; English witness (n.) "one who testifies," originally "testimony." Martyr complex "exaggerated desire for self-sacrifice" is attested from 1920.